1. Field of the Invention
The invention in general relates to a temperature monitoring system for an electric generator and particularly to a generator having water cooled stator coils.
2. Description of the Prior Art
In electric generators such as those driven by steam turbines, a tremendous amount of heat is produced during normal operation. Some generators may weigh hundreds of tons and the unchecked production of heat in such machines as caused by mechanical and I.sup.2 R losses would quickly lead to a complete generator failure.
Accordingly, the heat dissipation requirement for a generator is accomplished with a cooling system which utilizes a flow of cooling gas within the generator housing as well as within the rotor and stator structures to remove the produced heat. Some generator designs additionally flow the cooling gas through the stator coils themselves while in another design a cooling liquid such as water is passed through the stator coils for cooling purposes. In the water cooled system, water is piped into a circumferential manifold at one end of the generator and flows into the stator coil ends by means of tubing. The water discharges from the stator coil at the opposite end where it is collected by tubes feeding into a discharge manifold from which the water is processed and cooled for recirculation. During passage through the stator coil, the water functions to absorb generated heat.
A plurality of temperature sensors are provided for the continuous monitoring of the cooling water to protect the generator against failure. A rise in temperature of the cooling water may be indicative of such problems as a broken conductor or a coolant water flow reduction due to a partial blockage of the coolant path.
The generator stator core, made up of a plurality of thin laminations, has equally spaced slots running the entire length of the core. Each slot is deep enough to accommodate two separately wound coil sections and the water discharge from each coil section is monitored by a respective temperature sensor such as a thermocouple to obtain a temperature indicative reading.
A common method for monitoring the thermocouples consists of forming two separate groups, the top coils in the slot constituting one group and the bottom coils of the slots constituting a second group. The high to low temperature difference between temperature sensor readings in each of the two groups is calculated separately and if the temperature spread between the highest and lowest reading is above a certain value, a warning indication is provided to the operator and in response to which the generator may be removed from service.
For a 48 slot stator, there would be 96 stator coil temperature sensors. Periodic recordation by hand of 96 sensor readings and a calculation of a high-low difference is an extremely time consuming and laborious task. Accordingly, more advanced systems include means for periodically scanning the temperature sensors and placing the readings into the storage of a microcomputer which calculates the high-low difference. Since all temperature sensors are not identical, nor identically placed, a large temperature difference between the high and low readings may exist even under normal operating conditions and especially at high generator load levels. In addition, as will be subsequently explained, a temperature sensor during a malfunction may provide an abnormally high reading from its previous normal reading, however, such condition will go undetected utilizing the high-low difference method between coil groups.
The present invention provides for an improved temperature monitoring system for such generator wherein early detection of an abnormally hot stator coil is made possible independent of the effects of variations in inlet water temperature, water flow rate and generator load conditions.